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Window decorations are handled by the window manager and typically include the window title bar, window title bar buttons and window borders -- and those are the only things a window manager theme can change.

A GTK+ theme on the other hand can change how things look inside the window of a GTK+ application: buttons, entries, labels and all other widgets get their visual style from the GTK theme.

This separation may sound arbitrary and useless... and in a way it is. It only exists because of the way X works: the X server draws the decorations for all windows and clients only draw the window contents. This may be simpler or at least different in the glorious future when X is replaced by Wayland (or Mir if that's the way you lean).